The present invention relates to a method and device for monitoring blood pressure. In particular, such method and device is non-invasive to the human body and the device is preferably portable.
In Singapore alone, there is at least one person coming down with stroke every hour. The numbers are rising year after year. Moreover, death from stroke in Singapore accounts for more than 12% of all deaths since 1996. Together with heart ailment, it accounts for more than 32% of all deaths since 1996, ie. more than one-third of all mortalities in Singapore.
Further, every year there are about 27,000 to 30,000 pregnancies leading to successful deliveries. Of these, thousands of pregnant women suffer from a condition called pre-eclampsia. This is a condition whereby the mother suffers from a rise in blood pressure during pregnancy. The blood pressure can rise to dangerous levels without warning and it can lead to convulsion and brain damage to the mother, and sudden intra-uterine death of the baby. The morbidity and mortality of pre-eclampsia is directly related to the level and control of blood pressure of the patient.
The central event linking the 3 major ailments is blood pressure. In fact, in many instances of strokes and heart attacks, the usual and final pathway is a sudden and dangerous rise in blood pressure before catastrophe strikes. Therefore, the detection and prevention of further rises or falls in the final pathway holds the key to the prevention and reduction of strokes, heart attacks and eclampsia.
Currently, patients who suffer from the above illnesses are monitored either as outpatients or in-patients in a hospital. The majority of these are outpatients. When one visits a doctor, be it monthly or fortnightly, the blood pressure reading is obtained by using a blood pressure cuff sphygmomanometer. They use occlusive methods, i.e. air is pumped into the cuff to occlude the artery and is slowly released to finally allow the blood to overcome the resistance and flow through. A flow turbulence is thus set up and picked up by the doctor listening to it. The blood pressure is then recorded. The self-monitoring devices that are available on the market generally all use occlusive methods, the difference being the turbulence are picked up by various methods, such as via a microphone. In other words, the number of readings is totally dependent on the number of times that the artery is being occluded, whether it is manual or pre-set electronically. The monitoring is therefore not continuous, in the sense of having beat-to-beat readings.
To make matters worse, whenever the doctor detects a normal or xe2x80x9cgoodxe2x80x9d blood pressure in his clinic, he usually makes 3 assumptions:
1. the patient""s blood pressure from the last test must be xe2x80x9cgoodxe2x80x9d;
2. his blood pressure until the next test will be xe2x80x9cgoodxe2x80x9d;
3. therefore, he will not have a stroke, heart attack or convulsion as in the case of a pre-eclampsic woman.
Unfortunately, these assumptions are far from the truth as the above incidents have revealed. Therefore, it would be advantageous to be able to catch the xe2x80x9cfinal pathwayxe2x80x9d of sudden changes in blood pressure/pulse, by being able to monitor a person""s blood pressure continuously and be able to sound the alarm at the right time to prevent a catastrophe.
One method of continuously monitoring blood pressure is suggested in U.S. Pat. No. 5,485,848. That patent purports to disclose a non-invasive and non-intrusive portable device for monitoring a user""s arterial blood pressure. However, that device has the disadvantage that it needs to fix a nominal or base pressure by fixing the strap tension. The calibration is also user-specific. It assumes that base pressure can be maintained constant for the calibration to work. It is not practically possible to fix the base pressure of a moving wrist by the methods described. At most, it only keeps the strap circumference constant, instead of keeping the pressure constant. By fixing the circumference of the strap, pressure changes are even greater with movement and changes in position of the hand. Thus, the wrist position cannot change. In practice, it is difficult to keep the pressure constant as a slight change in wrist pressure and sensor position affects readings to an appreciable extent. Furthermore, the calibration involves extrapolation and interpolation of readings. Therefore, user conditions must remain uniform, since one has to show a linear relationship which may not exist if user conditions are otherwise. In accordance with the described formula for calculating blood pressure, the pressure sensed by the piezo-electric film transducer is dependent on the area of contact, distance from the artery and source of the signal. These are factors which cannot practically be fixed with the described device.
To provide continuity in monitoring, the blood pressure must be measured on a beat-to-beat basis, as in intra-arterial monitoring.
The time-keeping function of a watch should be integrated with the blood pressure data, as this will provide a meaningful interpretation of the trend or pattern of blood pressure seen or recorded over a period of time. The downloading of data over time may become important in an unfortunate event of the death of a wearer.
Similarly, in the collection of data by the sensor, the position of the sensor and the fixation of the sensor must be considered. In order to accurately collect data from every beat of the heart, the sensor compartment must be able to receive reliable data with the wrist in different positions. In the prior art, the data can only be reliably collected when the hand is held fixed at a certain position, i.e. with restrictions. The prior art may try to overcome the movement of the strap by increasing the strap pressure. Usually, this is not only impractical, but undesirable as the compression of veins will cause significant congestion in the hand distal to it in just a few minutes. This can lead to numbness and further medical complications.
The Median Nerve at the Carpal Tunnel would be compressed causing numbness of the finger in a few minutes. As a result, the hand or fingers will swell, causing further congestion. This not only greatly affects the signal, but is harmful to the wearer. Therefore, the challenge is to be able to design the strap system that is comfortable to the wearer over a long period and holds the sensor in position well so as to allow for natural movement of the hand/wrist and collects the data accurately.
The donning and doffing of the wrist monitor and the whole calibration has to be simple and user-friendly for it to be of value for a person who is not medically trained.
Against this medical background and clinical deficiency, the problem is to provide an improved device and method for continuous and non-invasive monitoring of arterial blood pressure. Such a device should preferably be capable of alarming the user of harmful rises or falls in the user""s blood pressure.
According to one aspect of the present invention, there is provided a device for continuously monitoring a user""s arterial blood pressure, including:
sensor means adapted to continuously detect said blood pressure and to generate signals representative thereof by contact with an external surface of the user""s body at a location adjacent an artery;
attachment means for securely holding the sensor means in operable contact with the user""s body at the said location; and
microprocessor means for interpreting said signals generated by the sensor means to determine actual arterial blood pressure;
wherein the sensor means includes a projecting portion for detecting and transmitting changes in blood pressure, and wherein the projecting portion is adapted to effect at least partial occlusion of the artery at the said location.
In a preferred embodiment of the invention the attachment means is adapted to non-penetratingly press the projecting portion into the surface of the user""s body for operable contact therewith at said location adjacent the artery.
In a preferred embodiment of the invention the senor means includes a transducer and the projecting portion of the sensor means is adapted to transmit detected changes in blood pressure to the transducer. Preferably, the projecting portion of the sensor means is a dome-shaped plunger connected to the transducer.
According to another aspect of the present invention, there is provided a method for continuously monitoring a user""s arterial blood pressure, including the steps of:
providing sensor means adapted to continuously detect said blood pressure and to generate signals representative thereof by contact with an external surface of the user""s body at a location adjacent an artery;
maintaining the sensor means in operable contact with the user""s body such that said sensor means effects at least partial occlusion of the artery at the said location; and
computing the actual arterial blood pressure detected by the sensor means using microprocessor means adapted to interpret the signals generated by the sensor means.
In a preferred embodiment of the invention the step of maintaining the sensor means in operable contact with the user""s body includes the step of non-penetratingly pressing a projecting portion of the sensor means into the surface of the user""s body.
In a preferred embodiment of the invention the step of maintaining the sensor means in operable contact with the user""s body includes the step of securely holding the sensor means at the said location by attachment means such as a strap.
In developing a blood pressure monitoring device according to a most preferred embodiment of this invention, there are various design considerations which should be met. A truly effective device for continuous blood pressure monitoring should meet the following basic requirements:
1. Portability.
2. Continuity.
3. Accuracy in the calibration, collection of data and ability to function in a natural environment without interruption to daily activities.
4. User-friendly so that there is no need for medically-trained personnel to operate the device.
5. It must not cause any other medical complications
6. It should advantageously be useable also as a communications tool in order to manage the data collected.
It will be convenient to hereinafter describe the invention in greater detail by reference to the accompanying drawings which illustrate one particularly preferred embodiment. The particularity of the drawings and the related description is not to be understood as superseding the generality of the broad identification of the invention.